1. Field of the Invention
The present invention relates to a composite solder glass, particularly a composite solder glass with a reduced melting temperature, and a filling material for the solder glass containing doped lead titanate. The composite solder glasses obtained with this filling material are suitable for hermetically sealing, soldering and coating individual components made from glass, glass-ceramic material, ceramic material and metal. Their preferred applications are for devices and components in the electrical engineering and electronics fields. The invention also relates to a method of making these kinds of composite solder glasses and filling materials.
2. Prior Art
Generally a composite solder glass comprises a powder mixture that contains a solder glass powder with a reduced melting temperature and a substantially inert filling material powder for adjustment of the thermal expansion properties. Conventional solder glass powder contains PbO and B.sub.2 O.sub.3 as the principal ingredients, in particular cases ZnO, F, SiO.sub.2, Al.sub.2 O.sub.3, Bi.sub.2 O.sub.3 and other conventional glass ingredients are used. Other low melting solder glass powders are based on PbO, V.sub.2 O.sub.5, TeO.sub.2 or other glass systems. PbO--B.sub.2 O.sub.3 -Solder-Glass generally has a thermal expansion coefficient of about 9 to 12.times.10.sup.-6 /K, measured between room temperature and 300.degree. C. The lower melting temperatures are desired in order to keep the thermal stresses for the elements to be bonded or coated small and in order to shorten the processing times for cooling to room temperature from the maximum temperature. During the melting organic auxiliary materials are evaporated, which e.g. are used as suspension agents for application of the composite solder glass. The filling powder comprises one or more filling materials, which should be insert in comparison to the glass powder, so that damaging interactions do not occur, such as dissolution of the filling material by the solder glass, gas release or uncontrolled crystallization of the solder glass. Conventional filling materials for lowing the thermal expansion of composite solder glasses are, e.g., .beta.-eucryptite, cordierite, mullite, willemite, zircon, aluminum oxide and/or lead titanate. The selection of the filling material depends also on the intended use. When good electrical insulation properties are required, alkali-containing filling materials are usually avoided, such as .beta.-eucriptite. Additional description of composition, properties and uses of glass solder is found in "GLASS SCIENCE AND TECHNOLOGY", Uhlmann, N.J. Kriedl; Chapter 6, pp. 169 to 207; Copyright 1984 by Academic Press. Inc.; "Glaslote (Glass solder)", G. Muller, in Glasshutten Handbuch (Glass Smelting Handbook), Z101/1-6, Copyright 1975 by Huttentechnische Vereinigung of Deutschen Glassindustrie (Smelting Engineering Associated of the German Glass Industry); Engineered Materials Handbook, Volume 4, "Ceramics and Glasses", by ASM INTERNATIONAL, Copyright ASM INTERNATIONAL 1991, Selection 14, p. 1069, "Sealing Glasses" (by Carl J. Hudecek).
In certain cases also additive ingredients, such as ZrO.sub.2, ZrSiO.sub.4 or TiO.sub.2, are added to the composite-solder glass power, which lead to the desired crystallization of the solder glass powder after melting. These crystallizing composite solder glasses, above all, are of advantage in later applications in which a higher thermal load resistance is desired.
The most important requirements of a composite solder glass are a lowered or reduced melting temperature and the adjustment of the thermal expansion property to the element or material to be coated or bonded over the entire temperature range below the freezing point of the composite solder glass to room temperature or the application temperature. The freezing temperature designates the temperature under which relaxation of thermal stresses caused by differing thermal expansion can no longer occur. The thermal expansion property of the composite solder glass is adjusted to fit the material or piece to be coated or bonded. Strength critical parts of the bonding can be assisted when there are insignificant differences in the thermal expansion coefficients during cooling under compressive stress. This compressive stress can compensate an exteriorly acting tensile stress and thus increase the strength. The freezing temperature is 10 to 30.degree. C. above the transformation temperature of the solder glass at the conventional cooling speeds. Additional important requirements for the composite solder glass are good adherence to the material to be coated or bonded, hermetic sealing ability and high strength for the composite.
The matching of the thermal expansion properties over the entire temperature range under the freezing temperature is important in order to avoid temporary tensile stresses. The resulting stresses impair the strength if a mismatch occurs. Cracks or tears which endanger the hermetic seal or the mechanical bond arise immediately or during use if an even larger mismatch occurs.
Known filling materials based on lead titanate and on doped PbTiO.sub.2 -mixed crystals, in which a part of the Pb or Ti is replaced by other cations, can be characterized as follows in regard to their thermal expansion properties. Until at a certain temperature, the Curie temperature T.sub.c, they have a lower often even negative thermal expansion. Above this temperature, at which also the ferroelectric properties also vanish, they have a positive thermal expansion. In order not to impart this discontinuity in the thermal expansion property of the filling material to the composite solder glass, the freezing temperature of the composite solder glass should not substantially exceed the Curie temperature of the filling material. Otherwise larger temporary stresses between the composite solder glass and the pieces or materials to be coated or bonded can hardly be avoided. The Curie temperature amounts to about 490.degree. C. in pure PbTiO.sub.3. The Curie temperature is strongly reduced by a series of cation substitutions, such as the substitution of Pb by Ca. This can make this type of cation substitution unusable for composite solder glasses with higher freezing temperatures.
An additional disadvantage of the filling material based on PbTiO.sub.3 caused by cationic substitution is that the degree of substitution must be changed within wide limits in order to vary the thermal expansion properties of the filling material. Greater variation of the thermal expansion of the filling material is often only possible, when the type of cation substitution, i.e. the element connected with it is changed. Thus it is important that the composition of the filling material must always be compatible with the glass solder contained in the composite solder glass. An additional risk of impermissible reactions arises then from the new elements introduced by the cationic substitution. Also certain elements are often not allowed because of the requirements of the applications or the processing. Thus e.g. usually alkali atoms must be avoided for applications involving contact with Si as a semiconductor or which require high electrical insulation properties. Furthermore it is economically disadvantageous to prepare a filling material for a wide range of thermal expansions and applications which has many different cations. Because of logistic reasons such as storage, supply and mutual contamination danger it is disadvantageous that the manufacturing conditions must be adjusted to the respective cationic substitution.
The reduced thermal expansion of PbTiO.sub.3 and PbTiO.sub.3 -mixed crystals is defined by the thermal expansion coefficient of the individual crystal axes. The thermal expansion properties often show a distinctive anisotropy. For example, in the case of tetragonal PbTiO.sub.3 the a-axis has a weakly positive thermal expansion and the c-axis has a strongly negative thermal expansion. Because of the thermal expansion differences the stresses between the filling material part and the solder glass can lead to damaging microscopic flaws, which can impair the hermetic seal properties and the strength.
German Patent Document DE-A 25 48 736 describes the glass passivation of the silicon semiconductor component. The passivation glass used therein has a reduced melting temperature. If necessary fine grained, undoped lead titanate is mixed in it as a filling material. A matching of the filling material to the thermal expansion properties by substitution of individual cations or anions is not described in this reference.
In Japanese Patent Application JP-A 04-160 035 many cationic substitutions for Pb and especially Ti atoms are described. This plurality of substitutions shows that the crystals of PbTiO.sub.3 have a distinctive ability to form mixed crystals at the cationic sites. Thus Pb is replaced up to 5 to 40 atom % by one or more of the elements Ca, Ba and Sr. The Ti atom is replaced up to 5 to 90 at % by Zr, Mg, Co, Zn, Ni, Mn, Sn, Cu, Fe, Al, Bi, Nb, Ta, Sb, Te, V, Mo and W. The glass containing the composite solder comprises PbO, V.sub.2 O.sub.5, TeO.sub.2 and according to choice or optionally Al.sub.2 O.sub.3, ZnO and Nb.sub.2 O.sub.5. The substitution by the named cations permits adjustment or matching of the thermal expansion properties of the obtained filling material and its Curie temperature. The use and the manufacture of individual filling materials with clearly different chemical composition is however expensive and involves the above-described technical and economical disadvantages.
German Patent Document DE-A 37 12 569 describes a cation-substituted filling material of the formula: EQU (Pb.sub.1-m Ca.sub.m)TiO.sub.3, wherein 0&lt;m.ltoreq.0.40.
That means that up to 40 atom % of Pb are replaced by Ca. The filling materials are used for sealing elements in electronics components. The substitution of Pb by Ca leads to a strong reduction of the Curie temperature, combined with a reduced value for the thermal expansion of the filling material. The presence of microscopic flaws after sealing in glass is disadvantageous.
German Patent Document DE-A 39 11 176 describes an attempt to eliminate the disadvantages of microscopic flaws in the filling material by using a filling material composition comprising 65 to 75% by weight PbO, 10 to 25% by weight TiO.sub.2, 1 to 10% by weight Fe.sub.2 O.sub.3, 1 to 12% by weight WO.sub.3 and 0 to 5% CaO. In this composition the Pb is preferably replaced by a Ca atom and the Ti preferably by a combination of Fe and W. Composite glass solder made with the filling material of the claimed composition has a thermal expansion coefficient between 3.5 and 5.0.times.10.sup.-6 /.degree.C. between room temperature and 250.degree. C., also in a comparatively limited range. The filling materials are especially suitable for is obtaining a reduced thermal expansion coefficient in composite solder glass. The Curie temperature is strongly reduced with the claimed cation substitution to a value of about 300.degree. C., which makes the named filling materials unusable for many composite solder glasses with higher freezing temperature.